In most trenching operations in soft soil, the cross-section of the excavation area turns out to be of generally trapezoidal shape. Although the hoe or similar excavation means will initially remove only a rectangular section of soil, almost invariably the upper edges of the trench so dug will then cave in and leave the trapezoidal form mentioned above. A wider horizontal section of soil is therefore customarily removed from the top than is required by the dimensions of the bottom of the trench. The same phenomenon necessitates the use of substantial areas outside and alongside the trench for positioning pile driving means in order to reinforce the walls of the excavated ditch. Because such piling is usually driven in perpendicular to the bottom of the trench, the top section of each pile is freestanding and not supported by the surrounding soil. It must therefore be driven in a substantial distance beyond the bottom of the trench in order to assure the required structural rigidity for the sidewalls thus formed.
There are many situations, however, where the room for operating trenching and piling equipment is severely restricted, both horizontally and, possibly, even vertically, be it because of problems of access and right-of-way or because of geological conditions and where control of grade and line is important. In such circumstances the desirability of digging and reinforcing trenches with truly vertical sidewalls and with a minimum of lateral surface access requirements has long been recognized by the industry. See, for example, U.S. Pat. No. 4,279,548, entitled "Hydraulic Propulsion Unit", issued July 21, 1981, to Raymond W. Ramey.
It is the object of the present invention to satisfy this long felt need and the need for speed of excavation and uniformity of trench and operation with the help of a hydraulic, self-advancing excavation unit which requires the removal of, and access to, no more soil than is actually defined by the dimension of the trench while using a propulsion system anchored in front of the unit to produce a trenching machine quality, uniform trench in conjunction with standard trenching tools, such as a back hoe.